Tropical plankton exodus by 2100

HORDES of microscopic plants could be driven out of tropical oceans over the course of this century by rising sea temperatures. The consequences for fisheries and those who rely on them could be severe, as these phytoplankton are an important food source in marine ecosystems.

As Earth's climate warms over the coming decades so will the oceans. To find out how phytoplankton will contend with higher temperatures, Mridul Thomas of Michigan State University and colleagues looked at different species' optimum temperatures for survival. They found that many tropical species are already living right at, or even slightly above, their optimum temperature, making them vulnerable to any increase.

The team then simulated ocean temperatures predicted to occur in the 2090s if we continue emitting greenhouse gases, to estimate how the phytoplankton's habitats would change. They found that a third of tropical species were forced to the cooler waters of the poles where they would have a better chance of survival. "You see this big drop in diversity in the tropical oceans," Thomas says (Science, doi.org/jm2).

It's not clear how ecosystems will change in practice, says Thomas. In theory, the remaining phytoplankton species could become more common, in which case animals like fish will still have plenty to eat.

Experience suggests otherwise, says Stephen Simpson of the University of Exeter in the UK, who has shown that warming reduces ocean productivity. "If this becomes the norm for the tropics, crucial ecosystems will deteriorate and fisheries will fail," he says.

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